I've recently been having problems with my '92 LS400. A little history first: 97k miles, Always had problems with overheating, probably put more engine coolant than gas! air suspension has been known to not pump up and leave me stranded wherever. But other than that I love this lexus and it has gotten me to and from work for 3+ years.
Recently however the speedometer and rpm's dont work. But more importantly the check engine and trac off light will come on and when i attempt to accelerate say through a traffic light, the car will shutoff and not start back up leaving me helpless in a busy intersection. I pull the car over call for help and by the time it arrives the vehicle will start up again. The Idle will be all over the place and sometimes after stopping at a stop sign I wont even have to push the gas, the car will accelerate and sometimes i have to barely touch the gas in fear that if I gun it, it will die again.
I took it to the stealership and 15mins later received a call that it was my alterator and p/s pump was leaking all over it. So i replaced the alternator and rebuilt the p/s pump and guess what? exact same problem.
I cleaned out the throttle body, checked wires and harness's my ect sensor tested good once and bad once but havent replaced that yet.
Running a diagnosis with the tdcl under the steering wheel gave me trouble code 22 ect sensor circuit and my o/d off light blinked code 42 which i believe is vehicle speed sensor circuit.
I know i cannot trust the dealership, they took me for my $100 diagnosis and kicked me out the door with a bogey analysis.
Please help I know you CL members are the best!!!

 

While your situation is very frustrating, don't completely disregard the dealership. If you feel that you have been taken, give the service manager a call. Let him know that you came in for a specific problem, and while they may have corrected malfunctioning parts on your vehicle, they did not address your concern. You were put under the impression that the alternator was causing this problem, and approved the repair as you wanted to fix the idle/cutting off issue. It does sound like you may have a ECT issue. However, will be tough to know as the vehicle is quite old. It could be more than a number of things causing it. I've seen broken distributor buttons do weird things. The Tach/speedo problem is common. Usually requires the replacement of both of the components. Speed sensors are hard to tell which are bad considering you won't be able to read a data list on a scan tool. Start with the ect sensor cuz if it is intermittently shorting out will definitely lead to intermittent idle problems especially if the vehicle thinks it's cold when it's hot.